1. Field of the Invention
The present invention relates to an apparatus for coding a video signal in video communication, video transmission, video storing, and broadcasting and, more particularly, to a video coding apparatus using motion compensation predictive coding.
2. Description of the Related Art
A TV telephone, a TV meeting system, an optical disk apparatus, a VTR, a CATV, and the like require a technique for coding a video signal. As such a video coding scheme, so-called motion compensation prediction coding is known. In this scheme, a pixel value of a picture to be coded (to be referred to as a to-be-coded picture hereinafter) is predicted by using a pixel value of a coded picture designated by a motion vector, and a corresponding predictive error and the motion vector are coded.
Assume that such a motion compensation predictive coding scheme is applied to an interlaced video (field picture) signal. In this case, the scheme is not suitable for a motion precision higher than an intra-field 1/2 line, e.g., a higher precision than an intra-frame one line, because there is no corresponding pixel value in a reference picture.
For this reason, a method of performing motion compensation by interpolating a pixel value of a corresponding pixel which does not exist on a reference picture using the pictures of the two adjacent fields has been proposed (e.g., "Adaptive Line Interpolated Inter-field Motion Compensation Method", Image Coding Symposium, 1990, (PCSJ90), 8-1). In this motion compensation method, a to-be-coded picture is coded by using a reference picture and an optimum motion vector. The reference video signal is formed by interpolation using a signal, of coded video signals of past two fields, which is located at a position designated by a motion vector obtained by a motion vector searching circuit. More specifically, three field memories are prepared, and a signal obtained by performing intrafield interpolation using an output from the first field memory is mixed with an output from the second field memory at a mixing ratio of km:1-km. The value km varies depending on the motion magnitude detected by a motion magnitude detection circuit on the basis of outputs from the first and third field memories.
According to this conventional technique, an interpolation value is formed by using the video signals of two adjacent fields in accordance with the motion magnitude so that an appropriate reference video signal corresponding to a motion precision higher than an intra-field 1/2 line (a motion precision higher than an intra-frame 1 line) can be generated for a field picture, thereby allowing high-precision motion compensation predictive coding.
In this scheme, however, the motion between two reference pictures must be detected, as described above, and hence a motion magnitude detection circuit is required. In addition, in order to perform motion magnitude detection, the pictures of three adjacent fields must be coded before the detection. If the pictures of three adjacent fields are not coded before motion magnitude detection, the detection cannot be performed.
In a conventional video coding apparatus using the above-described motion compensation predictive coding scheme, when a search for a motion vector for motion compensation is performed in a forward or backward direction, a reference picture for searching for the motion vector is limited to one coded picture in a case that a to-be-coded picture is a non-interlaced video. For reasons of this, accurate motion compensation cannot be performed with respect to a video which moves between adjacent pictures in units of 1/2 pixels.
Of the above-described video coding schemes, a video coding scheme having a transmission rate of about 1 to 2 Mbps has been developed to be a standard, which is termed "MPEG1" , for a picture storage such as VCRs and optical disks. This scheme is based on motion compensation inter-frame prediction and DCT (Discrete Cosine Transform).
A scheme for coding a video having high quality equal to or higher than quality for TV broadcasting at about 2 to 10 Mbps has been studied for the same purpose as described above. A coding scheme of MPEG1 is designed to be applied to a non-interlaced video as input signals. However, since the standard TV signal is interlaced video, where the coding scheme MPEG1 is applied to the interlaced video, a new means suitable for interlaced video is required. An inter-field/inter-frame adaptive prediction scheme is known as a coding method of interlaced video. In this scheme, a field having the same scan phase as that of a coding (to-be-coded) field (an odd field when an odd field is coded and vice versa), and a field having a scan phase different from that of the coding (to-be-coded) field and close in time thereto (e.g., an even field when an odd field is coded and vice versa) are switched as a prediction signal. In addition, interpolation prediction has recently been studied, which forms prediction signals by averaging signals extracted from current and previous fields (e.g., F. Wang et al., "High-quality coding of the even fields based on the odd fields of interlaced video sequences" , IEEE trans. CS).
When an interlaced video is subjected to a predictive coding using current and previous fields as in the coding scheme MPEG1, the even and odd fields suitable for the interlaced video is applied to a prediction. In this case, since the amount of motion vector information is increased when motion vectors are sent for the respective fields, means for decreasing the amount of motion vector information without a decrease in efficiency is required. That is, it is required to improve the prediction precision with respect to an interlaced video and decrease the information amount of predictive error coded outputs. In addition, it is required to minimize an increase in motion vector information. However, no effective techniques capable of satisfying such requirements have been proposed yet.
As described above, in the conventional technique, in order to interpolate between the pixels on a reference picture using two field pictures adjacent to the reference picture, motion magnitude detection is required for the reference picture. Therefore, a motion magnitude detection circuit is required, and the hardware inevitably becomes complicated. In addition, if three adjacent fields are not coded before motion magnitude detection, the detection cannot be performed.
Furthermore, according to the conventional technique, since a reference picture is limited to one coded picture in a search for a motion vector, accurate motion compensation cannot be performed with respect to a video which moves between pictures in units of 1/2 pixels. Further, if a prediction signal is formed referring to plural frames, since a large amount of arithmetic operation is required to search for a motion vector, the motion vector search time is prolonged or the circuit size of the hardware is increased.
Moreover, in the conventional technique, the prediction precision with respect to an interlaced video cannot be effectively improved, and the amount of motion vector information sent for the respective fields is undesirably increased.